The National Transportation Safety Board today issued two
urgent safety recommendations to the Federal Aviation
Administration (FAA). The first recommendation asks that the
FAA require operators of aircraft equipped with a particular
model engine to immediately perform blade borescope
inspections (BSI) of the high pressure turbine rotor at
specific intervals until the current turbine disk can be
redesigned and replaced with one that can withstand the
unbalance vibration forces from the high pressure rotor. The
second recommendation asks the FAA to require the engine
manufacturer to immediately redesign the disk. The NTSB
issued an additional recommendation for a requirement that
operators perform a second type of inspection and another
recommendation related to the engine manufacturer regarding
the installation of the replacement disk.
All four recommendations apply to the low pressure turbine
(LPT) stage 3 (S3) rotor disk in the General Electric (GE)
CF6-45/50 series turbofan engines that can fail unexpectedly
when excited by high-pressure (HP) rotor unbalance.
An uncontained engine event occurs when an engine failure
results in fragments of rotating engine parts penetrating
and exiting through the engine case. Uncontained turbine
engine disk failures within an aircraft engine present a
direct hazard to an airplane and its passengers because
high-energy disk fragments can penetrate the cabin or fuel
tanks, damage flight control surfaces, or sever flammable
fluid or hydraulic lines. Engine cases are not designed to
contain failed turbine disks. Instead, the risk of
uncontained disk failure is mitigated by designating disks
as safety-critical parts, defined as the parts of an engine
whose failure is likely to present a direct hazard to the
aircraft.
In its safety recommendations to the FAA, the NTSB cited
four foreign accidents, which the NTSB is either
investigating or participating in an investigation led by
another nation, in which the aircraft experienced an
uncontained engine failure of its GE CF6-45/50 series
engine.
The date, location, and circumstances of these four events
(none had injuries or fatalities) are as follows:
On July 4, 2008, a Saudi Arabian Airlines (Saudia) Boeing
747-300 experienced an engine failure during initial climb
after takeoff from Jeddah, Kingdom of Saudi Arabia. This
investigation has been delegated to the NTSB.
On March 26, 2009, an Arrow Cargo McDonnell Douglas DC-10F,
about 30 minutes after takeoff from Manaus, Brazil,
experienced loss of oil pressure in one engine. The pilots
shut down the engine and diverted to Medellin, Columbia.
This investigation has been delegated to the NTSB.
On December 17, 2009, a Jett8 Cargo Boeing 747-200F airplane
was passing through 7,000 feet above ground level (agl) when
the flight crewmembers heard a muffled explosion and
immediately applied left rudder. With one engine losing oil
pressure, the airplane returned to land at Changi,
Singapore. The NTSB is participating in the investigation
that is being led by the Air Accident Investigation Bureau
of Singapore.
On April 10, 2010, an ACT Cargo Airbus A300B4 experienced an
engine failure while accelerating for takeoff at Manama,
Bahrain. The crew declared an emergency, rejected the
takeoff, activated the fire suppression system, and
evacuated the airplane. The NTSB is participating in the
investigation that is being led by the Bahrain Ministry of
Transportation – Civil Aviation.
The four recommendations to the FAA are as follows:
1. Immediately require operators of CF6-45/50-powered
airplanes to perform high pressure turbine rotor blade
borescope inspections every 15 flight cycles until the
low pressure turbine stage 3 disk is replaced with a
redesigned disk that can withstand the unbalance
vibration forces from the high pressure rotor.
(Urgent)
2. Require operators of CF6-45/50-powered airplanes to
perform fluorescent penetrant inspections of CF6-45-
50- low pressure turbine stage 3 disks at every engine
shop visit until the low pressure turbine stage 3 disk
is replaced with a redesigned disk that can withstand
the unbalance vibration forces from the high pressure
rotor.
3. Immediately require General Electric Company to
redesign the CF6-45/50 low pressure turbine stage 3
disk so that it will not fail when exposed to high
pressure rotor unbalance forces. (Urgent)
4. Once General Electric Company has redesigned the CF6-
45/50 low pressure turbine (LPT) stage 3 disk in
accordance with Safety Recommendation [3], require all
operators of CF6-45/50-powered airplanes to install
the newly designed LPT S3 at the next maintenance
opportunity.